Method of and apparatus for increasing the accuracy of electrochemical grinding process



Nov. 4. 1969 KIYOSHI INOUE METHOD OF AND APPARATUS FOR INCREASING TH OFELECTROCHEMICAL GRINDING PROCE 5 Sheets-Sheet 1 Filed Dec. 5, 1966-INVENTCR K/ran/ /4/0(/E ATTORNEY N V- 4. 1969 KIYOSHI INOUE 3 6,

.ME'IHOD OF AND APPARATUS FOR INCREASING THE ACCURACY OF ELECTROCHEMICALGRINDING PROCESS Filed Dec. 5, 1966 5 Sheets-Sheet 2,

/a3 A I INVENTOR 44%? :w/ Nous as j I BY ATTORNEY Nov. 4. 1969 KIYOSHImout METHOD OF AND APPARATUS FOR INCREASING THE ACCURACY OFELECTROCHEMICAL GRINDING PROCESS Filed Dec. 5, 1966 5 Sheets-Sheet s IFIG. 7

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Gnm :m:m:E|:::1-- 3//' g 326 f INVENTOR 1 v k/rasu/ M 0015 7 BY g. -R 7ATTORNEY Nov. 4, 1969 KIYOSHI INOUE 3,476,662

METHOD OF AND APPARATUS FOR INCREASING "IHE ACCURACY OF ELECTROCHEMICALGRINDING PROCESS 5 Sheets-Sheet 4 Filed D80. 5. 1966 3 v Q 5 5 a 8rakeew? a INVENTOR K/ V0.5 M

ATTORNEY Nov. 4. 1969 I KIYOSHI lNOUE 3,

METHOD OF AND APPARATUS FOR INCREASING THE ACCURACY OF ELECTROCHEMICALGRINDING PROCESS 5 Sheets-Sheet 5 Filed'Dec. 5. 1966 name-y UnitedStates Patent U.S. Cl. 204-143 2 Claims ABSTRACT OF THE DISCLOSUREImproved electrochemicalgrinding apparatus and method whereby theaccuracy of reproduction of the electrode contour in the workpiece issharply increased by limiting the thickness of the electrolyte filmcarried by the grinding wheel to a minimum. The film thickness isreduced by a scraper or wiper held against the wheel, by directing astream of high-velocity air thereagainst to dissolve the excesselectrolyte, by suction removal of the excess, and/or byelectrochemically reforming the electrolyte.

The wiper may serve as an electrode for the reformation of theelectrolyte film or as the nozzle for the jet of air and is contouredcomplementarily to the wheel by electrochemical action or castingthereagainst. Both the wheel and the electrode are preferably composedof graphitic materials.

The present invention relates to a method of and an apparatus forincreasing the accuracy of electrochemicalgrinding (ECG) techniques and,more particularly, to improvements in electrochemical grinding affordinga more accurate shaping of the workpiece, reduced tool wear, lower powerconsumption, and better workpiece finish.

Electrochemical-grinding techniques have been in use for many yearsinasmuch as electrochemical grinding is characterized by an excellentworkpiece finish and an ability to machine metals of various hardnesses.In earlier electrochemical-grinding systems, a generally conductive tool(e.g. a metal-bonded diamond wheel) and the workpiece are urged togetherwith diamond particles or other means forming a miniscule interelectrodegap through which an electrolyte is entrained as a film on the rotatingtool electrode.

In my copending patent applications Ser. Nos. 512,338; 535,268, now U.S.Patent No. 3,417,006; 562,857, now U.S. Patent 3,420,759, and 565,670,filed respectively Dec. 8, 1965, Jan. 19, 1966, July 5, 1966, and June30, 1966, I have disclosed and claimed improvements in theelectrochemical grinding whereby the disadvantages of diamond-containingwheels and other means for maintaining the slight interelectrode gap canbe avoided. As pointed out in these applications, when the electrode andthe electrolyte have approximately the same specific resistivities, onecan dispense with diamond or other particles as interelectrode spacersinasmuch as the electrolyte film inherently present in theinterelectrode gap and/or filling irregularities in either of theelectrode surfaces conducts a substantial portion of the machiningcurrent so that electrochemical erosion is carried out in spite of atendency for the workpiece to directly contact the electrode.

3,476,662 Patented Nov. 4, 1969 It has been found that substantially allelectrochemical grinding methods are characterized by disadvantageswhich are only partly understood but apparently derive from the natureand character of the electrolyte in the interelectrode gap and upon thesurfaces of the electrode and the workpiece. Thus, when a high-speedrotating tool electrode is used to shape a conductive workpiece and anelectrolyte flows along the periphery of the tool, undercutting atcorner, angular or edge portions is a common occurrence. Suchundercutting frequently 'exceeds the machining tolerances of the systemand renders electrochemical grinding unsuitable for certain purposes.Investigations have demonstrated that there is a connection between thedegree of undercutting and the characteristics of the electrolyte uponthe rotating tool. More specifically, it is believed that thecurrent-carrying electrolyte tends to move away from the tool surface athigh speeds under the centrifugal action of the rotating grinding wheeland against the surface-tension forces tending to hold the liquidelectrolyte against the wheel. When ridges or grooves are to be formedin the workpiece by a correspondingly profiled wheel, therefore, theroots of the grooves tend to become channeled. Because machining takesplace preferentially at the roots of the grooves, the ridges are exposedto correspondingly less machining current and may be unaffected by theelectrochemical grinding action.

It is, therefore, the principal object of the present invention toprovide a method of and an apparatus for increasing or improving theaccuracy of electrochemical grinding (ECG) techniques.

A further object of this invention is to provide an improved method ofand apparatus for extending the principles originally set forth in thecopending applications mentioned earlier.

Yet another object of this invention is to provide apparatus of theelectrochemical-grinding type which will afford an accuracysubstantially better than any possible heretofore.

Yet another object of this invention is to provide a method ofelectrochemically grinding metallic workpieces aifording high accuracy,improved surface finish, greater machining speed and reduced electrodewear.

Still another object of the invention is to provide an apparatus forelectrochemically grinding workpieces to impart complex profiles theretowith accurate reproduction of the tool profile.

These objects and others which will become apparent hereinafter areattained, in accordance with the present invention, by a method basedupon my surprising discovery that it is possible to improve manifold theaccuracy of electrochemical grinding techniques by controlling thenature of the electrolyte film directly upon the tool electrode. Theterm controlling when applied to the electrolyte upon the tool electrodeor grinding wheel is intended to refer to modification of thecomposition of the electrolyte as well as to alteration of its physicalcharacter directly upon the electrode surface whereby, when the improvedelectrolyte is disposed within the interelectrode gap, there issurprisingly reduced tendency to undercut the workpiece, to irregularlyerode the juxtaposed surfaces, etc.

According to a more specific feature of the invention and one particularaspect thereof, my process involves the stripping of excess electrolytefrom the electrode surface just prior to the juxtaposition of thisrotating surface with the workpiece for machining action. By strippingof the electrolyte, I intend to refer to the removal of all electrolyteexcept a thin film or a substantially monomolecular layer which remainsadherent to the surface by surface-tension and like forces.

Stripping of the electrolyte can, in accordance with the presentinvention, be carried out in different ways, depending upon theconstruction of the electrode, etc. Thus, I have found that, especiallywhen the electrode is deeply profiled, it is ossible to remove excesselectrolyte and form upon the machining surface of the tool electrode arelatively thin film or monolayer of electrolyte by urging against thissurface a complementarily profiled squeegee in the form of a relativelystiff thin foil. The foil, which can be of Celluloid, polyethylene,polytetrafiuoroethylene or other low friction synthetic resin(uncorrodable by the electrolyte) may exert a wiping action against thesurface or function as a liquid scoop to lead the excess electrolyteaway from this surface. Alternatively, a thin conforming film ofelectrolyte may be ensured by closely juxtaposing with the machiningsurface a suction head adapted to siphon or aspirate excess electrolytefrom the rotating machining face. It has been found that regardless ofthe degree of suction, sufficient electrolyte remains adherent to permitthe electrochemical grinding action although practically allundercutting is eliminated. In addition to or instead of the use ofsuction, it is also contemplated, in accordance with the presentinvention, to direct a jet of highvelocity gas against the machiningsurface, thereby dislodging the excess electrolyte.

According to another aspect of this invention, the electrochemicalgrinding action is markedly improved by electrically transforming theelectrolyte upon the tool electrode surface prior to entrainment of theelectrolyte film into juxtaposition with the workpiece. In accordancewith this aspect of the invention, an auxiliary electrode member can bejuxtaposed with the machining surface so that the film of electrolytebridges the gap between the auxiliary electrode and the tool electrode,while circuit means applied the modifying current between this auxiliaryelectrode and the tool electrode. It has been found to be highlyadvantageous to provide, as the auxiliary electrode, a dispensing heador nozzle for the electrolyte stream, thereby ensuring that a conductivepath is provided through the electrolyte jet supplied to the machiningsurface.

While the mechanism for the electrical transformation of the electrolytefilm is not fully understood in its effect upon the machining accuracyof an electrochemical grinding action, it may be hypothesized that someform of the electrolytic decomposition, ionization, or the like occursin the film, which transformation has a recombination or restorationtime less than that required for entrainment of the transformedelectrolyte into the interfacial regions. It is also conceivable thatthe alternatingcurrent energization (and especially high-frequencyelectrolyte activation) gives rise to a disruption of any polarizationeffects or to an activiation at the electrolyte/electrode interfacefacilitating the uniform flow of machining current between the toolelectrode and the workpiece. This modification at theelectrode/electrolyte interface may, moreover, improve the adhesion ofthe electrolyte to the tool electrode, thereby reducing undercutting. Atany rate, a surprising increase in machining accuracy is observed whenthe film of electrolyte is treated directly upon the rotating machiningsurface by applying thereto a direct current, an alternating current, ahigh-frequency alternating current, (e.g., from the l kilocycle to the 3megacycle range) or direct current upon which an alternating current ofthe low-frequency (e.g., 30-500 cycles per second) or high-frequency(e.g., kilocycle or megacycle range) is superimposed.

The present invention has been found to be especially suitable for usewith the systems described in the aforementioned copending applicationwherein a graphite electrochemical-grindirv Wheel is employed with anelectrolyte having a dynamic specific resistivity approaching that ofthe graphite wheel. In practice it has been found that the machiningcurrent can be direct current or alternating current, the latterapparently being effective because of the preferential erosion of theworkpiece.

According to still another aspect of the present invention, themachining accuracy of the system can be improved by modifying thesurface tension or surface-tension characteristics of the electrolytefilm upon the machining surface. More particularly, I have found it tobe advantageous to add one or more organic surfactantforming compoundssuch as olive oil, asphalt oil, stearic acid, caproic acid, cetylalcohol or the like which, when subjected to electrical modificationwith the auxiliary electrode mentioned above and/or when combined withthe ions in the electrolyte tend to form true metal soaps orsurface-active agents in situ. The presence of the surfactants appearsto reduce the tendency of the electrolyte to migrate away from the toolelectrode and thus serve to reduce undercutting in the manner previouslydescribed. The surfactant-forming compounds are preferably longchainorganic alcohols, acids or alkyl, aromatic or aralkyl oils.

Experiments have demonstrated that the techniques of the presentinvention, when applied to electrochemical grinding of complex profiles,toothed or serrated articles and the like can increase the machiningaccuracy up to or greater than 3 /2 times and yield better surfacequalities. In fact, the improvements described above makeelectrochemical grinding competitive with most die-making and machiningsystem as far as accuracy is concerned.

The above and other objects, features and advantages of the presentinvention will become more readily apparent from the followingdescription, reference being made to the accompanying drawing in which:

FIGS. 1 and 2 are diagrammatic cross-sectional views illustrating theflow characteristics of electrolyte upon the periphery of rotating ofelectrochemical grinding wheels prior to this invention;

FIG. 3 is a cross-sectional view illustrating the undercutting which mayresult from the flow characteristics of FIGS. 1 and 2;

FIGS. 4 and 5 are diagrammatic elevational views illustrating improvedelectrochemical grinding systems in accordance with the presentinvention;

FIG. 6 is a cross-sectional view taken generally along the line VI-VI ofFIG. 5;

FIGS. 7 and 8 are diagrammatic and somewhat elevational views of otherembodiments of the invention;

FIG. 9 is a graph showing results obtainable with the device of FIG. 8;

FIGS. 10 and 11 are diagrammatic elevational views of grinding systemsusing the fiat surface of a wheel;

FIG. 12 is a fragmentary perspective view showing another feature ofthis invention;

FIG. 13 is a diagrammatic and partially sectioned elevational view; and

FIG. 14 is a perspective cross-sectional view of another grinding systemillustrating the principles of the present invention.

In FIGS. 1 and 2, I show diagrammatically the nature of an electrolytefilm F along the machining face M of a graphite wheel W used for theelectrochemical grinding of a workpiece (not shown) in accordance withany of the systems set forth in my copending applications mentionedabove. The profiled wall W is formed with a series of ridges R, R" andR' along the machining face with intermediate troughs T and T. Ridge Ris of wedge-shaped section and tends to cast off the electrolyte film Fin a relatively extended sheet F whereas the trough R" flattened so thatsomewhat shorter sheets of electrolyte F" are cast off in the radialdirection. A rounded surface such as that of the ridge R forms stillsmaller disk-like streams F' of electrolyte. When the machining face Mclosely approaches a workpiece as illustrated at w, for example, in FIG.3, these spurious streams of electrolyte F to F'" appear to give rise toundercutting of the channels machined into the workpiece w at U, U" andU'. Thus the profile of the machine surface differs materially from theprofile of the contoured electrode.

As illustrated in FIG. 2, the disadvantages described above alsocharacterize smooth surface wheels such as that at U. Here the machiningface M is cylindrical although the electrolyte film is again nonuniformand is cast off in radial sheets F etc. When this wheel is used tomachine a workpiece with any of the systems illustrated in theaforementioned copending applications or even with conventionalelectrochemical grinding apparatus, surface irregularities are found inthe workpiece in spite of the relative smoothness of the tool electrode.In fact, I have discovered that the roughness and inaccuracy in thesurface of the workpiece derives in large measure from the nature andcharacter of the electrolyte film.

The disadvantages discussed in connection with FIGS. 1-3 can be avoided,in accordance with one aspect of the present invention, by removingexcess electrolyte from the machining face of the tool electrode and/orby applying the electrolyte thereto in such manner that a thin film ofelectrolyte or a monolayer only is formed uniformly along the electrodesurface. In the system of FIG. 4, a workpiece is shown to be supportedon a crossfeed carriage 11 of conventional construction and to bejuxtaposed with an electrochemical grinder wheel 12 (e.g. of graphite).The wheel 12 is used to form a channel 13 in the workpiece and for thispurpose is connected to an electrochemical grinding power supply adaptedto apply direct or alternating current across the wheel 12 and theworkpiece 10 against which the wheel is urged. The wheel is preferablyconstructed in the manner set forth in my copending application Ser. No.565,670 of June 30, 1966, while the electrolyte recirculation apparatus,the power supply and the feed systems can be any of those described andillustrated in my applications Ser. No. 512,338 of Dec. 8, 196 5, and562,857 of July 5, 1966. A power supply of this type is represented at14 in FIG. 4 and has one terminal connected to a wiper 14' which, inturn, engages the shaft 12 of the grinding wheel 12. The other terminal14" is connected with the workpiece 10.

The grinding wheel 12 has its shaft 12' driven by a motor 12" and isformed with a cylindrical machining face 15 to which the electrolyte issupplied via a nozzle 16 close to the point at which the machiningsurface 15 rises from the workpiece 10. The electrolyte-coated machiningsuface is carried in the direction of arrow 17 (i.e. clockwise) past awiper 18 of a porous material to which a suction pipe 19 is connected. Anegativepressure blower or suction pump 29 communicates with the line19' so that the head 18 simultaneously wipes the electrode surface 15and sucks excess electrolyte therefrom through the porous body 18. Anapron or guide 21 below the wiper 18 conducts the squeegeed liquid awayfrom the maching face. After the endless and continuous machining face15 sweeps past the spongelike suction wiper 18, a relatively thin filmof electrolyte remains upon the surface. This thin film may be furtherreduced by a jet of high-pressure air represented at 22 and directedfrom a nozzle 23 generally tangentially against the machining face 15counter to its direction of rotation. I have found that such a jet doesnot decrease the uniformity of the film in the sense that one mightexpect but rather markedly improves the machining accuracy. In fact, thejet 22 may be used alone in the event the head 18 is omitted. A blower24 delivers compressed air to the nozzle 23. A further suction nozzle 25is open toward the machining zone to collect any electrolyte which mightotherwise tend to accumulate there, thus ensuring that only the thinfilm will be entrained by the tool electrode 15 through the machiningzone. The workpiece 10 is displaced in the usual manner on its crossslide or the like in the direction of arrow 26 to produce the channel13. The duct 19 can be provided with a three-way valve 27 to which anelectrolyte-circulating pump 28 and a reservoir 29 of electrolyte areconnected when it is desired to feed the electrolyte into the systemthrough the porous applicator 18. In this case, only the blower 23functions as a means for removing the excess electrolyte.

In FIGS. 5 and 6, I show another system for removing electrolyte fromthe surface 115 of a contoured graphite electrochemical grinding wheel112. A nozzle 116 here delivers the electrolyte to the machining face115 and the excess is removed by a thin synthetic resin foil 130 whoseedge 131 is complementarily contoured to inter fit with the ridges1151", 115r" and 115r of the wheel 112. The foil 130 is turned againstthe wheel 112 so as to form a scoop along which the excess electrolyteflows away from the machining face 115 prior to its engagement with theworkpiece to form a complementary profile 113 therein. Only a monolayeror thin film of electrolyte remains on the lower right-hand quadrant ofthe claimed wheel 112 as the machining surface is brought into contactwith the workpiece or juxtaposed therewith adjacent this quadrant. Thefoil 130 can conduct electrolyte to a reservoir 129 from which it issupplied, via the usual filters and the like, to the nozzle 116 via apump 128. In practice, it has been found that the major part of theelectrolyte applied to the surface of the wheel 112 is recovered at 129so that only a thin film remains upon the machining surface asindicated. Replenishment of the electrolyte is carried out via a supplyreservoir 132 whose valve 133 feeds additional electrolyte into thecirculating system in proportion to the volume of the electrolyte filmwithdrawn from this system. Of course, it is also possible to provide acollecting vessel for recoverable electrolyte which may run off theworkpiece and, in this case this fraction of the electrolyte is filteredand returned to the circulating path as well. It has occasionally beenfound to be economical to discharge this portion of the electrolyte,however, which sustains the greatest contamination. The thin film at themachining face resulting from the wiper action of the foil completelyeliminates undercutting as discussed in connection with FIGS. 1-3,presumably because the electrolyte flow pattern F, F" etc. no longerresults and the electrolyte clings closely to the electrode surface.

In another modification of the present system as illustrated in FIG. 7,the workpiece 210 is fed in the direction of arrow 226 on the usualcross slide or carriage 211 while the electrochemical grinder wheel 212is coated with electrolyte from a porous block 218. Excess electrolyteis removed from the machining surface 215 of the wheel by directing asheet-like jet of air thereagainst counter to the direction of rotationof the wheel (arrow 217) from a nozzle 223 Whose mouth 223' is theentire width of the machining face 215. Here the nozzle 223, which isconnected to the blower or compressor as described in connection withFIG. 4, is located substantially at the lower right-hand quadrant andnot more than 90 ahead of the workpiece or machining zone in the angularsweep of the surface 215. Also in this zone, I provide a suction head225 whose suction aperture 225' is closely spaced from the surface 215while spanning the entire width of the latter. A pump 228 circulateselectrolyte connected at the suction nozzle 225 and connected from theworkpiece 210 to the pipe 219 supplying the electrolyte-dispensingwiper. In this case, too, the presence of only a thin film ofelectrolyte at the working zone 233 appears to ensure greatly improvedaccuracy and freedom from undercutting of the type described earlier.

According to another aspect of this invention, the electrolyte iselectrolytically transformed on the workpiece surface with the aid of anauxiliary electrode just in advance of contact between the transformedfilm and the workpiece such that the recombination time of thetransformation is substantially less than the time required to sweep thetransformed portion of the electrolyte to the machining interface. Forexample, it may be assumed that the electrolyte consisting of asodium-chloride solution is electrically altered to promote theformation of NaOH, ClOH, HCl, NaO- or other species known to begenerated in the electrolysis of sodium chloride, sodium nitrite,potassium nitrite and kindred alkali-metal salts. These species havevarious recombination times or decomposition periods leading towardrestoration of the simple K+, Na+, Cl-, N 1 NO and hydrated-ioncondition of the electrolyte. It appears that the aforedescribedelectrically transformed electrolyte is capable of eliminatinginaccuracies resulting from undercutting of grooves and overcutting (inthe sense of excess material removal). Greater reproduction accuracy ofthe contours of the electrochemical grinding wheel is obtained. It maybe noted parenthetically that at least part of the effect of electricaltransformation of the electrolyte layer on the tool electrode may derivefrom ohmic heating of the electrolyte and vaporization thereof to leavea reduced film thickness thereon.

In FIG. 8, I show an electrochemical grinder having an electrolytecollection trough forming a receptacle 311 in which the metallicworkpiece 310 is disposed. The trough 311 can be shifted in thedirection of arrow 326 via a diagrammatically illustrated lead screw 311and feed motor 311". The graphite electrochemical grinding wheel 312 issupported in a conventional head and urged against the workpiece 310under sping or fluid pressure in the direction of arrow 334 (asdescribed and illustrated in the aforementioned copending applicationsdealing with electrochemical grinding). The shaft 312 of the wheel 312is driven by the motor 312" in the direction of arrow 317 so that themachining surface 315 sweeps past a discharge nozzle 318 deliveringelectrolyte to the face 315. A recirculation system represented by thepump 328 and the pipe 319 delivers electrolyte to the nozzle 318. Thepower supply 314 connected across the workpiece 310 and the wheel 312 isof the type disclosed in the above-mentioned copending applications andcan be direct current, direct current superimposed upon alternatingcurrent or even alternating current. According to the presentimprovement, the nozzle 318 forms an auxiliary electrode whose face 318'is closely juxtaposed with the machining face 315 of the tool electrodeso that the electrolyte film between them forms a bridge and iselectrically transformed directly upon the machining face. For thispurpose, a further power supply 340 is connected between the wheel 312(via its shaft 312' and a brush 314') and the auxiliary electrode 318.As will be apparent hereinafter, the power supply 340 may be adirect-current source so poled that the auxiliary electrode 318 isrelatively positive or relatively negative or an alternating currentsource. The passage through the auxiliary electrode 318 thus deliversthe electrolyte to the machining face.

In FIG. 9, I show a graph of the machining characteristics as a functionof the current applied between the workpiece electrode 312 and theauxiliary electrode 318. In FIG. 9, the electrolyte transformationcurrent is plotted along the abscissa while the machining rate(broken-line curves) in grams per minute and the corner radius(solidline curves) in mm. are plotted along the ordinate. A series ofthree graphs are provided in each set for the auxiliary electroderelatively negative and relatively positive (D.C. supply) and for analternating-current auxiliary supply. The corner radius obtainable withthe auxiliary electrical modification of the electrolyte (with AC. orDC.) is a marked improvement over any obtainable without suchmodification although a relatively positive auxiliary electrode providesthe greatest reduction in corner radius with increase of current flow.In all cases, the accuracy increases sharply initially and tends tolevel out with increasing auxiliary current and at the uppercurrentamplitude ranges, no substantial further loss in machining speedis observed; the machining rate generally increases although a decreaseis observed when the auxiliary electrode is relatively positive.

In FIG. 10, I show a modified electrochemical current system wherein aworkpiece 410 such as a tool bit or the like is ground between theannular face 415 of a graphite electrochemical grinding tool of the typedescribed and illustrated in my application 565,670 filed June 30, 1966.The workpiece 410 is mounted on the carriage 411 and is urged in thedirection of the wheel 412 (arrow 426) via fiuidor spring-pressure meansand the usual leadscrew. The electrochemical-grinding power supply 414is represented as a transformer connected to an A0. line and functionsas set forth in the copending applications mentioned above. Here,however, the electrolyte is delivered from a reservoir 429 via a pump428 to the nozzle 418 which also serves as an auxiliary electrode toelectrically transform the electrolyte film. For this purpose, theauxiliary power supply connected between the electrode 418 and thegrinding wheel 412 comprises an alternating current source 440acapacitively bridged across a direct-current source 44Gb whoseinductance 4400 limits current surges. The capacitor 440d is connectedin series with the alternating-current source and the slider or wiper ofa potentiometer 4402 in series with the auxiliary electrode 418 and thetool electrode 412. The potentiometer 440e thus can be adjusted tocontrol the amplitude of the alternating current superimposed upon theDC electrolytetransforming current passing through the electrolyte filmbridging the auxiliary electrode 418 and the tool electrode 412. Again,a substantial improvement in machining accuracy is obtained.

In the modified system of FIG. 11, the auxiliary electrode 518a isconstituted by a graphite wiper spanning the radial width of thetransverse annular machining surface 515 of the graphite wheel 512. Atool bit or other workpiece 510 to be shaped is urged in the directionof arrow 526 against the face 515 and ECG machining current is appliedagainst the workpiece 510 and the tool electrode 512 via a source 514.The wiper 518a thus serves to mechanically remove excessive electrolyteand as the auxiliary electrode for its electrical transformation. Theauxiliary current source 540 connected between the auxiliary electrode518a and the tool electrode 512 across the electrolyte film, isconstituted of an AC. generator or line 540a connected via an isolationtransformer 54012 and a D.C.-blocking capacitor 5400 between theauxiliary and tool electrodes. A voltmeter 551 is in parallel with theAC. source 540 to measure the amplitude of the auxiliary voltage.

A servomechanism 550, responsive to voltage fluctuations, is connectedacross the auxiliary electrode 518a and the tool electrode 512 toregulate the flow of electrolyte to the dispensing nozzle 518b via theelectromagnetically controlled valve 519. The circulating pump 528delivers electrolyte to this valve from the reservoir 529. When theelectrolyte film on the surface is of the proper thickness andcharacter, it possesses a predetermined resistance so that a voltagebelow a predetermined peak is sensed by the servomechanism 550. When,however, the resistance rises between the auxiliary electrode 518a andthe tool electrode 512, the servomechanism 550 operates the valve 519 tosupply more electrolyte to the face 515 and thus correct the filmthickness. When excess electrolyte is present, the reduced resistance isalso sensed by the mechanism 550 which reduces the supply of electrolyteto restore the optimum film thickness and character.

In the systems of FIGS. 8-11, I have found it advisable to addlong-chain alcohols, organic acids and organic oils to the electrolyteundergoing electrical transformation in the film upon the machiningsurface; it appears that the electrolytically produced species orfragments (e.g. KOH, NaOH and ClOH) may react chemically with thesesurfactant-forming organic compounds to stabilize the electricaltransformation and produce surface-active agents which also favorablymodify the characteristics of the electrolyte in the sense that animproved machining accuracy is obtained. In the system of FIG. 8, forexample, the auxiliary electrode 318 must be located at, say, 10 abovethe machining zone for most machining speeds unless a transformationstabilizer is provided. Of course, the angle can be approximatelydoubled when the peripheral speed of the wheel is doubled, theconsideration being the recombination rate' of the electrochemicallyproduced species.

The following examples illustrate how the present invention can becarried out in practice.

EXAMPLE I Using the apparatus illustrated in FIGS. and 6, a graphiteelectrochemical grinding wheel having a diameter of 180 mm. and aspecific resistivity of 3.4 ohm-cm. and a width of 20 mm. was used togrind a 0.55% (by weight) carbon steel (855C) workpiece. The machiningface of the wheel had a serrated profile with 4 teeth with apex anglesof 60 each and flank heights of 3.5 mm. The peaks were, therefore,spaced apart from one another 'by 3.5 mm. The peripheral speed of themachining face of the wheel was 22.5 m./ second and the electrolyteconstituted of an aqueous solution containing 3% by weight sodiumnitrite and 5% by weight potassium nitrate. The pressure applied to theelectrode in urging it against the workpiece was 0.1 kg./cm. while themachining power was 7 volts peak-to-peak (50 cycles A.C.) with a meancurrent of 50 amps.

A foil 130, shaped to be complementary to the configuration of theserrated wheel, was held as illustrated in FIG. 5 with a squeegee gap of0.2 mm. The foil was 0.1 mm. thick polytetrafluoroethylene (Teflon).With the foil, an accuracy at the apexes of the teeth of 0.02 mm. wasobtained whereas the corner radius was 0.07 mm. when the foil wasremoved. The machining speed was approximately the same in both cases.

EXAMPLE H The apparatus illustrated in FIG. 4 was employed to machine atungsten carbide workpiece containing 6% cobalt. The electrode was asimple cylindrical wheel having a serrated periphery as described inExample I and was urged against the workpiece with a pressure of 1.5kg./cm. The machining current of 80 amps. was delivered at 6 volts peakto peak (50 cycles per second A.C.). The machining depth was 5 mm. andthe machining speed held at l.61.8 mm./min. When no means was used totreat the film on the electrode surface, the roughness was found to beabout 0.5 micron H and the corner radius 0.3 mm. When, however,compressed air was directed against the tool electrode via the nozzle 23at a pressure of 6 kg./cm. and a nozzle/electrode gap of 1 mm.,machining could be carried out at the same rate with the improved cornerradius of 0.08 mm., an accuracy of 0.015 as compared with 0.07 mm. ofdeviation from the contours of the tool electrode.

EXAMPLE III Using the apparatus of FIG. 8, SK-Z die steel was machinedwith a graphite wheel having a 1 cm. machining area and a diameter of150 mm. The speed of the machining surface was 23.2 m./ second and 750cc./min. of electrolyte was delivered to the machining zone. Theelectrolyte was an aqueous solution containing 2% by weight sodiumnitrite and 5% by weight of potassium nitrate; the machining current was70 amps of direct current. The results obtained with various auxiliarycurrents without the addition of surfactant formers are graphed in FIG.9. When the auxiliary current was 100 amps (between the auxiliaryelectrode 318, disposed 10 above the machining zone, and the toolelectrode 312), the machining rate was approximately 0.7 g./ min. whilea corner radius of about 0.006 mm. was observed when the electrode 318was relatively negative. When the auxiliary power supply was alternatingcurrent, a machining speed at this auxiliary current flow of 0.5 g./min.and a corner radius of about 0.01 mm. were noted. When the electrode 7was relatively positive, an auxiliary current of amps gave about 0.005mm. corner radius and -a machining speed of about 0.38 g./min. When theauxiliary current is not applied, the corner radius is invariablygreater than 0.03 mm. Thus it can be seen that it is possible toincrease the machining rate and accuracy when the auxiliary electrode isrelatively negative, to increase the accuracy at the expense of themachining rate when this electrode is relatively positive, and toincrease the accuracy without any substantial modification of themachining rate when an alternating current is used as the auxiliarysupply.

It is possible to increase the accuracy still further (i.e. to have thecorner radius) when a surfactant former is supplied to the electrolytein the amount of 0.5% by weight, the surfactant former consisting of along-chain organic compound soluble in the electrolyte at least uponelectrical modification thereof. 0.5% solutions of stearic acid, caproicacid, cetyl alcohol, olive oil and asphalt oils in the salt-containingelectrolytes described yielded approximately half the corner radiusobtained without the surfactant former for the same auxiliary-currentflow.

Referring now to FIG. 12, it can be seen that a contoured grinding wheel612 of graphite, carried by an arbor or shaft 612 and driven by asuitable motor for the grinding of a workpiece in systems similar tothose of FIGS. 4 and 6-8, for example, co-operates with a wiper 630whose front end 630a is tapered forwardly in the direction of theelectrode 612 so that an ECG power supply 614 can be connected betweenthe graphite wiper 630 and the wheel 612 to electrochemically machinethe scrap er 630 to contours 6311b complementarily to the contours ofthe Wheel 6112 by rotation of the later and the delivery of the usualmachining electrolyte to the region in which the scraper 630 is heldagainst the wheel. A spring force F urges the scraper 630 against thewheel and may be applied to the scraper by the system illustrated inFIG. 13 or any other convenient spring device. In this case, the scraper630 is machined as a workpiece to conform to the contours of the wheel612 prior to the machining process. In practice, therefore, the wheel612 may be contoured by conventional dressing means initially or bycasting in a mold and is thereafter used to contour the scraper 630 byelectrochemical machining. Subsequently, the scraper is positioned inplace of the scraper 130, for example, and machining of a workpiececarried out in the manner illustrated and described with respect toFIGS. 5 and 6. Since the forward end of the scraper 630 is tapered at630a and thins down in the direction of the wheel 612, electrochemicalmachining of the scraper to conform it to the contours of the wheel 612will be carried out preferentially with a relatively deep cut of thescraper and little, if any, erosion of the wheel.

In an alternative system, the scraper 630 can be formed by castinggraphitic material against the wheel whose contours thus determine thecomplementary contours of the scraper. As indicated above, the scraper630 is composed of graphite and may, therefore, serve as the auxiliaryelectrode for electrolytic transformation of the electrolyte. To thisend, the power supply 340 may be connected between the electrode 630 andthe wheel 612 or the workpiece (not shown) in the manner described inconnection with FIG. 8.

EXAMPLE IV Using a graphite electrochemical grinding electrode aspreviously described, with a specific resistivity in the radialdirection of about 3.4 10 9cm., a thickness of about 20 mm. and adiameter of about mm., electrochemical grinding of a workpiece composedof GB 885 tungsten carbide was carried out. The workpiece had a width of30 mm. and a thickness of 12 mm. The contours of the wheel were formedby four V-shaped grooves with a flank height and separation of 35 mm.The electrolyte was an aqueous solution of by weight potassium nitrateand was delivered to the wheel as illustrated in FIG. 5. The grindingwheel was rotated with a peripheral speed of 22.5 m./sec- 0nd and anelectrochemical grinding power supply was connected between the wheeland the workpiece as illustrated in FIG. 4, to deliver a grindingpotential of about 7 volts and a current of 50 amperes at a frequency of50 cycles/second. The scraper was a graphite plate and a synthetic resinfoil (of. FIGS. 12 and 6), respectively, in a series of tests. In thefollowing table, I list the results of five tests, comparing the maximumerrors obtained with no scraper, with the improved graphite scraper atscraper pressures (against the wheel) at pressures of 400, 800 and 2200grams/cmi, and a synthetic-resin-foil scraper of the type described inFIG. 6 at a pressure of 800 grams/cm for machining periods of 22 to 26minutes and a cutting depth of 5.5 mm.

From the foregoing results, it will be apparent that the accuracy atsimilar pressures of a graphite scraper contoured against the wheel isabout 2-4 times greater than that of the foil scraper system and may bemore than 6 times the accuracy obtaintable without a scraper or wiper.

In FIG. 13, there is shown another apparatus for carrying outelectrochemical grinding of a workpiece in accordance with the presentinvention, the wheel 712 here being provided with a hood which performsthe function of the suction nozzle 25 of FIG. 4. The hood 725 recoversthe electrolyte mist and returns it to the electrolyte source.Electrolyte is delivered by the nozzle 716 whereas a suction-type pickup725a is juxtaposed with the electrode surface to rapid removal of mostof the excess electrolyte. The scraper or wiper 730, which may beidentical to that of FIG. 12, is carried in a housing 760 in which aspring 761, whose force is adjustable by a screw 762, bears upon a seat763 of the wiper 730. The latter is guided by bearings 764 within thehousing so that substantially all of the spring force is effective tourge the wiper 730 against the wheel 712. A pipe 730, generally similarto the pipe 630' of FIG. 12, delivers a gas (e.g. air) under pressureand at high velocity to the interface between the wiper 630, 730 and therespective wheel 612, 712. As shown in FIG. 12, the wiper 630 is formedwith an internal cavity 665 communicating with the air inlet 630' andextending substantially to the tip of the wiper 630. Thus, when thecontours 63% are formed in the scraper 630, the chamber 665 opens at thecontact interface to permit the air jet to sweep away excesselectrolyte. I have found that this chamber 665 is best formed by a pairof graphite plates 666 and 667 formed with registering and confrontingrecesses and which are secured together by screws 668.

EXAMPLE V A tungsten-carbide workpiece, containing 6% cobalt, ismachined with a graphite electrode wheel whose specific resistivity inthe radial direction is about SZ-cm. The workpiece has an end face ofrectangular configuration with a width of 28 mm. and a height of 20 mm.The electrolyte is 5% an aqueous solution containing 5% by weightpotassium nitrate and the wheel has a peripheral speed of 20/mm. second,a diameter of about 25 cm. and a thickness of 28 mm. The machiningcurrent, applied as previously described between the wheel and theworkpiece is 8 volts alternating current at 50 cycles/ second. Prior tothe machining operation, the periphery of the grinding wheel iscontoured by a cutting tool. Thereafter, the electrode 630 or 730 isurged against the electrode and machined at 8 volts until the tip 630abottoms in the roots of the recesses of the wheel. The power supply isdisconnected from the electrode 630, 730 and is connected with theworkpiece. The scraper 630, 730 is unhollowed. When cutting theworkpiece to a depth of 8 mm., a maximum error of 0.008 mm. is found inthe reproduction of the profiles of the electrode surface in theworkpiece.

EXAMPLE VI The process of Example V is followed except that the scraperthere used is replaced by a hollow scraper 630 formed by boltingtogether a pair of plates (FIGS. 12 and 13). The end face of the scraperis of rectangular configuration with a height of 15 mm. and a width of35 mm. whereas the exposed chamber has a rectangular cross-section of 10mm. x 25 mm. The profile of the scraper is formed by electrochemicalgrinding for 45 minutes. The scraper is urged against the wheel with aconstant pressure of 3 kg./cm. while air is forced through the scraperat a pressure sufficient to permit the air to emerge at the interface.

As the gas pressure increases from 0 to 1 kg./crn. the accuracyincreases from tolerances of 50 microns to 1020 microns. Areproducibility of better than 10 microns is obtained with pressuresbetween 2 and 6 kg./cm. the interfacial gap formed by the gascorresponds at these pressures to less than 0.1 mm. It is found thatsubstantially higher accuracy can be obtained at low-material removalrates (by comparison with Example V, for instance) or that much highermachining rates (e.g. 0.8 mum/second) can be obtained with the sameaccuracy.

In FIG. 14, I show a modified system wherein a pair of scrapers 830a,830b is provided, each being of the double-plate hollow type illustratedin FIGS. 12 and 13 and being supplied with a respective gas-inlet tube830a and 8301) for delivery of air to the contact face. Both scrapers830a and 83011 are urged by springs such as the one shown in FIG. 13 inthe direction of the arrows F and F" against the electrode 812. Thedual-scraper arrangement of FIG. 14 is used to insure that a relativelythin film is maintained along the flanks 812; of the contours and anexcess electrolyte is removed therefrom. The plates 830a and 83% arepivoted upon respective shafts 880a and 88012 and provided with arms881a and 881b biased by the springs 882a and 8821; in opposite senses sothat the plate 838 is urged in the counterclockwise sense whereas theplate 83017 is urged in the clockwise sense to bring the edges 830ecloser to the flanks 812 of the wheel grooves. Thus, in spite ofpossible inaccuracies in reproducing the contours of the wheel in thescraper, the canting of the relatively thick scrapers brings thediagonally opposite edges of the contours thereof closer to the oppositeflanks of the grooves of the wheel and insures a minimum electrolytefilm thickness between the scraper and a machining surface at all pointsalong the machining face.

EXAMPLE VII A scraper for use in Examples IV, V and VI is prepared bymixing graphite and sulfur in a weight ratio of 1:2 to 1:4, melting themixture at a temperature between 180 C. and C., and thereafter castingthe melt in a mold bent around the contoured face of the electrochemicalgrinding wheel. Upon hardening, the plate was found to have a contourcomplementary to that of the wheel.

The invention described and illustrated is believed to admit of manymodifications within the ability of persons skilled in the art, all suchmodifications being considered within the spirit and scope of theappended claims.

I claim:

1. In a method of electrochemically grinding a conductive workpiecewherein a rotating tool electrode and workpiece electrode are broughttogether in the presence of an electrolyte at least at the interfacebetween said electrodes and an electric current is passed between saidelectrodes to erode electrochemically said workpiece electrode, theimprovement which comprises the steps of:

applying said electrolyte at least to a machining surface of said toolelectrode; and

increasing the accuracy of the electrochemical erosion of said workpieceelectrode by modifying the nature of the electrolyte film directly uponsaid surface to strip excess electrolyte from said machining surface ofsaid tool electrode prior to juxtaposition of said machining surfacewith said workpiece by applying suction through a suction head in closeproximity to said machining surface.

2. In an electrochemical grinding apparatus having a tool electrode witha rotatable machining surface, means for supplying electrolyte to saidmachining surface, and means for applying an electrochemical-grindingcurrent between said tool electrode and a workpiece brought intoproximity with said machining surface and constituted as a counterelectrode whereby said workpiece is eroded by electrochemical action,the improvemtnt which comprises means for modifying the electrolyte filmupon References Cited UNITED STATES PATENTS 3/1965 Williams 51-2731,062,248 5/1913 Mueller -1 51-273 2,739,935 3/1956 Kehl et al 2042242,899,781 8/1959 Williams 204224 2,939,825 6/1960 Faust et al. 2041433,008,892 11/1961 Owen 204224 3,061,529 10/1962 Crompton 204224 JOHN H.MACK, Primary Examiner SIDNEY S. KANTER, Assistant Examiner US. Cl. X.R.

